The present invention relates to methods and apparatus for logging earth formations penetrated by a borehole, and more particularly to methods and apparatus for measuring the density of downhole earth formations by means of gamma rays emitted from a logging tool into the formation and Compton scattered back to the tool.
In recent years nuclear well logs measuring the density of downhole earth formations have become increasingly important to petroleum engineers and log analysts. Porosities derived from density logs can be combined with measurements from resistivity or pulsed neutron logs to produce calculations of formation water saturations. Other combinations of density log data with that from sonic and/or neutron porosity logs can be used to determine formation lithology and to indicate formations with significant gas saturations. In certain areas even unsupplemented density log data can provide sufficient information to evaluate the formations of interest.
Density logging is based on the detection of attenuated gamma rays emitted from a radioactive source in a downhole tool. The gamma rays from the source penetrate through the toolcase, borehole, and formation. A fraction of these gamma rays are Compton scattered into and counted by one or more gamma ray detectors in the tool. The attenuation the gamma rays undergo between the source and detector(s) can, under certain conditions, be very simply related to formation bulk density. As a reasonable generalization, the count rate will decrease exponentially as the density of the formation/borehole system increases, and also as the source-detector spacing increases.
Gamma rays interact with matter principally by three processes: photoelectric absorption, pair production, and Compton scattering. Of these processes, only Compton scattering is not highly dependent upon the specific elements in the medium, depending instead upon only the density of the medium (the density being directly related to the number of electrons per unit volume). Photoelectric absorption and pair production, on the other hand, are strongly related to the atomic number Z of the nuclei in the formation and exhibit very strong gamma ray energy dependence. Therefore, current density logging tools are designed to respond only to Compton scattered gamma radiation, the selective response to such Compton scattered gamma radiation being achieved by proper selection of gamma ray energies and proper detector shielding.
Typically, present day density tools therefore measure density by observing an integrated Compton scattered gamma ray count rate over a broad, predetermined energy band. Since the higher density formation materials have higher gamma attenuation coefficients, the integrated count rate at the gamma ray detector in the logging tool will be lower when higher density material is present between the source and the detector. Using predetermined relationships, the count rates can then be converted into a measure of the formation density, and hence porosity. In some tools, two detectors are used to provide borehole (especially mudcake) compensation, but the measurement principle used in each detector is the same.
However, although some existing density logging tool configurations provide high quality measurements, a need still remains for improved Compton scattered gamma ray density measurement apparatus and methods. Preferably such methods and apparatus will offer density measurements, perhaps even with the same data generated by existing tools, but using different computational methods which are independent of the more conventional methods.